Proposal for the 252Cf source upgrade to the ATLAS facility Physics Division, Argonne National Laboratory Contact persons: Guy Savard, Richard Pardo February 22, 2005 Abstract Beams of accelerated exotic neutron-rich nuclei allow access to little known regions of the nuclear landscape that are important both structurally and for r-process nucleosynthesis. We propose to increase the radioactive beam capabilities of the ATLAS accelerator facility by the installation of a new source of ions to provide beams of short- lived neutron-rich isotopes. These isotopes will be obtained from a 1 Ci 252Cf fission source located in a large gas catcher from which the radioactive ions will be extracted and transferred to an ECR ion source for charge breeding before acceleration in the ATLAS superconducting linac. The technique is fast, universal and highly efficient. It will provide accelerated neutron-rich beam intensities of up to 7 · 105 ions per second on target at energies that are difficult to access at other facilities. This upgrade will enhance the reach of ATLAS and offer world-unique capabilities to study neutron-rich nuclei. It will also help advance technologies critical for the RIA facility. I. Summary Low-energy nuclear physics is at a very exciting time. The field, through both experimental and theoretical advances, has developed an “ab initio” understanding of the lightest nuclei starting from the nucleon-nucleon and 3-body forces, and an effective understanding of the heavier nuclei easily accessible in the laboratory. There is also a clear path to join these approaches from which an unified theory of most nuclei will emerge. Facilities, such as ATLAS, will play an important role in this quest. There are also indications from the region of very neutron-rich nuclei that the effective interactions are modified. This is the new frontier for low energy nuclear physics, where new phenomena are expected and a deeper understanding of so far untractable degrees of freedom will emerge. The research interests in the field are moving in this direction but will have to await RIA to reach the most remote regions. In the meantime, progress can be made with more limited facilities that will guide the way and help develop the techniques and expertise necessary to explore neutron-rich nuclei. Some neutron-rich radioactive beam capabilities exist at present facilities, but some of the requirements for a number of important studies are not met. We have taken a critical look at these requirements for basic classes of experiments and developed an upgrade plan for ATLAS that will address these issues. The plan is based on ion source and ion extraction techniques developed for RIA to be used in conjunction with a strong californium fission source. When combined with the high efficiency post-acceleration that ATLAS can provide, this will produce 2 beams of sufficient variety and intensity to address the core scientific questions. The upgrade plan is described in the following pages where it is demonstrated that the new technologies allow the important requirements to be met at a modest cost. The project is highly complementary to other efforts worldwide since the fission fragment distribution from californium is different from that from uranium and production is focused on nuclei that cannot be extracted by any of the present ISOL facilities, including HRIBF and ISAC, nor future ones, such as MAFF and SPIRAL2. It is a timely opportunity that has great physics and technical synergy with RIA and will help develop a map to guide the community in its future quests. 3 II. Table of Contents I. Summary 2 II. Table of Contents 4 III. Scientific Justification 6 A. Single-particle structure in the vicinity of magic nuclei 8 B. Pairing interaction in neutron rich nuclei 11 C. Gamma ray studies of neutron rich nuclei 13 D. Nuclear properties along the r-process path 17 E. Laser spectroscopy of neutron-rich nuclei 20 F. Stockpile stewardship 22 IV. Conceptual Overview 23 V. Technical Description 27 A. Source of radioactive isotopes 27 B. Gas catcher and degrader 30 C. Transport cask 35 D. RFQ gas cooler 36 E. Isobar separator 38 F. Beam dump 43 G. High voltage platform 44 H. Source region transport system and unaccelerated beam transport 47 I. Diagnostics station 47 J. Charge state breeder 48 4 K. ATLAS and diagnostics improvements 55 VI. Operations Issues 58 VII. Safety Issues 59 VIII. Budget 66 IX. Schedule and Manpower 70 X. Expected Performance 74 XI. References 82 5 III. Scientific Justification Our understanding of nuclear structure has evolved in stages, frequently driven by technological advances. Light ion induced reactions allowed the investigation of stable nuclei and the resultant explosion of new information stimulated the development of the shell model and collective models. Accelerated heavy ions allowed us to move away from the valley of stability and progress to very high spin. The curvature of the valley of stability allowed roughly a thousand new proton-rich isotopes to be studied. Again, this wealth of information stimulated theory and a new generation of mean field models and techniques for cranking the mean field to understand the effects of fast rotation. Now we approach a third phase. In this case theory and experiment are advancing together. In theory, the development of ab initio methods has moved our understanding of structure of light nuclei onto an entirely new quantitative plane with strong predictive power and high precision. In experiment, the challenge of very neutron-rich nuclei with completely new topologies such as neutron halos and skins has been glimpsed at, and accelerated radioactive beams are seen as the practical way to make progress. The neutron rich “terra incognita” in which thousands of isotopes lie, and about which we know little, has already been shown to be full of surprises. At the dripline, where binding is the weakest, extensive “halos” of low density neutron matter have been found in light nuclei. In several cases the dripline was found to extend further than expected. Nearer stability, strong modification to the normal sequence of single-particle states has been observed, leading to new shell gaps and new shapes. There are also strong indications, 6 from the isotope production in the r-process for example, that the pronounced shell structure we are familiar with close to stability is altered in weakly bound neutron-rich systems. Standard nuclear reactions tend to populate the proton-rich side of the nuclear chart and, as a result, the neutron-rich region of the nuclear chart has remained mostly uncharted. Exploring the far reaches of this region is a key component of the RIA scientific program. And while the full capabilities of RIA will be required to thoroughly explore this region, interesting forays in this new territory would yield extremely useful information provided intense neutron-rich isotope beams at Coulomb barrier energies were available. The californium source upgrade to ATLAS proposed here will provide an array of neutron-rich radioactive beams, including isotopes that have not been amenable to ISOL techniques before, at sufficient energy and intensity to provide a first glimpse at the key nuclear properties and help delineate some of the parameters required of the RIA research programs. The section below highlights the physics goals and proposed approaches for the initial investigations the californium source upgrade project will allow. The first four physics topics presented below can be investigated with the existing array of experimental equipment present at ATLAS, the fifth physics topic gives an example of new programs that could be initiated with modest investment (programs that could easily be initiated by users), and the last topic points out the unique capabilities that the ATLAS californium upgrade presents to study stockpile stewardship issues. 7 A. Single-particle structure in the vicinity of magic nuclei An important foundation of the description of nuclei is the characterization of the single- particle structure of stable nuclei near closed shells. These studies have provided critical information from stable nuclei on the ordering of single-particle states. Additional information near closed shells also provides the effective interactions between the nucleons in nuclei, that are the foundations of most modern nuclear models. It is expected that for the very neutron-rich isotopes the interactions will be modified by the neutron excess and the weaker binding and more diffuse nature of the neutron distribution. These changes could best be quantified by measurements of the single- particle structure on nuclei in the vicinity of closed shells in the neutron rich regions beyond stability. For short-lived nuclei, such measurements have to be performed in inverse kinematics with radioactive beams of energy well matched to the momentum transfer in single-nucleon stripping or pickup reactions. A prototype for such reactions are the neutron-adding stripping (d,p) or (α,3He) reactions with a 132Sn beam on a deuterium target. It is important in such studies to be able to determine the angular momentum transferred in the reaction and this requires that both the entrance and exit channels be well above the Coulomb barrier. Expected angular distributions for a neutron pickup with the Q value equal to that to the ground state of 133Sn with the above mentioned 132Sn(d,p) reaction are shown for different energies in Figure 1. It is seen there that the cross-sections increase rapidly above the Coulomb barrier and that the angular distributions become much more 8 distinctive. For the (d,p) reaction on Sn, the optimum energy is around 7.5 MeV/u. Pioneering work is being carried out at Holifield on this reaction at the sub-Coulomb energies 4.7 MeV/u that are an extremely useful first step.